Apparatus for making multiple point holograms

ABSTRACT

A method and an apparatus for producing point holograms by means of an array of pinhole diaphragms is described. By illuminating the array of pinhole diaphragms through an array of zone plates disadvantages associated with known methods may be avoided.

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Klotz 51 Aug. 14, 1973 APPARATUS FOR MAKING MULTIPLE [56] RelerencesCited POINT HOLOGRAMS UNITED STATES PATENTS [75] Inventor: Erhlrd PaulArtur Klotz, Hamburg, 3,405,614 10/1968 Lin et a1. 350/35 Germany3,545,854 12/1970 Oluon 350/162 Z? [73] Asaignee: U.S. PhilipsCorporation, New York, OTHER PUBLICATIONS N.Y. Sincerbox, IBM TechnicalDisclosure Bulletin," Vol.

[21] Appl. No.: 165,570 Primary Examiner-David Schonberg I}; a

Assirtant xaminerRonald J. Stem )1;

[30] Ford. Apphm mm Attorney-Frank R. Trifari July 24, 1970 Germany P 2036 904.4 A method and an apparatus for producing point holo- 521 US. Cl.350/35, 350/162 SF, 350/162 2?,

- 350/167 grams by of an array of prnhole dlaphragms 1s 51 Int. Cl.G021: 27/00 BY may [58] Field 6: Search 350/35, 162 R, 162 SF,disadva-mag associated with known methods may be avoided.

2 Chins, 2 Drawing Figures PAIENTED m l7! INVEN'IOR ERHARD PAUL ARTURKLOTZ AGENT APPARATUS FOR MAKING MULTIPLE POINT HOLOGRAMS The inventionrelates to a method of making point holograms in which the radiationemanating from an array of pinhole-diaphragms is made to interfere witha reference radiation and the resulting interference pattern is recordedon a photosensitive material. The invention also relates to an apparatusfor carrying out this method.

An important field of use of point holograms lies in the semiconductortechnology in which, as is known, such holograms may be used forproducing multiple images of masks used in the fabrication of integratedcircuits. Other uses lie in the fields of optical data processing,optical data storage and the production of synthetic holograms.

TWO FUNDAMENTAL METHODS ARE KNOWN a. Recording a point hologram of aminiature lens array (flys eye lens). However, these known lens arrayscan not satisfy the requirements of the semiconductor technology, Le, ahigh resolving power and. the production of high-quality images. Itwould appear that these images may be improved by using an array ofpinhole diaphragms which have diameters of about 1 pm and are arrangedin the foci of this lens array. Manufacturing such a pinhole diaphragmarray is hardly feasible owing to the large tolerances in themanufacture of the lens arrays. Manufacturing lens arrays havingimproved optical properties or smaller manufacturing tolerances is avery difficult and expensive process. At present lens arrays areavailable in a small number of dimensions only.

b. In another known recording technique the point hologram is made bystepwise displacement of a point light source and repeated exposure. Thepoint light source comprises a microscope objective of short focaldistance in the focus of which a pinhole diaphragm is located. The pointlight source is displaced by means of a slide rest. The spacing of theimaginary array of point light sources may be adjusted at will. In thisrecording technique only one point light source is required themanufacture of which presents no difficulty. In principle, however, thisstepwise production of the hologram has the large disadvantage that therepeated exposure involves a poor hologram efficiency. Owing to thestepwise recording this method takes comparatively much time.

A further possibility of making point holograms has been proposed in theprior Pat. application, Ser. No. 100570, filed Dec. 22, 1970. In thismethod the simplicity of recording and the good imaging properties ofthe stepwise recording technique are utilized, the poor hologramefficiency being counteracted by a subse-- quent holographic copyingprocess.

It is an object of the invention to provide an improved method ofproducing point holograms in which the disadvantages associated with thestepwise recording technique and the use of a lens array do not occur.The method according to the invention is characterized in that the arrayof pinhole diaphragms is exposed via an array of zone plates which isdesigned as a binary phase structure, the spacing of the array of zoneplates being equal to that of the array of pin-hole diaphragms.

The elements required for this method, i.e., pin-holes having diametersof about l pm and zone plates which in view of the desired low lightlosses must have a sufficiently large aperture ratio, may bemanufactured by using methods known in the semiconductor technology.These pinholes and zone plates may be imaged and multiplied by means ofa stepand -repeat camera.

According to the invention, to improve the optical density and theefficiency the photomasks obtained by the above method are furtherprocessed in the following manner. The photomask of the pinholediaphragm array is copied on a glass plate provided with a thin layer ofmetal by deposition from vapour, because the optical density of metallayers is better than that of photographic emulsions. For this purposeknown photographic-chemical methods are utilised which are also used inthe manufacture of integrated circuits. The zone plates, which initiallyare designed as amplitude structures, are unsuitable as illuminatingelements because of their poor efficiency. Hence the array of zoneplates is copied, for example, on a photoresist layer. The base for thephotoresist is a glass substrate. The zone plates obtained in thismanner as binary phase structures have an appreciably higher efficiency.The optimum modulation depth (It/2) may previously be determined by thechoice of the thickness of the resist layer.

The use of a stepand -repeat camera permits of choosing the spacing ofthe array of point light sources at will. The obtainable minimum spacingbetween the light sources is determined, however, by the geometricdimensions of the zone plates. Since, however, the zone plates mayoverlap one another, this spacing may be reduced.

Owing to the high reproducibility achievable by means of stepand -repeatcamera the array of pinhole diaphragms and the array of zone plates maybe made so as to have the same spacings, as is required for uniformillumination of the pinholes by means of the zone plates. At the sametime this manufacturing method ensures a constant geometry of thepinholes and the zone plates.

An embdiment .of the invention will now be described, by way of example,with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 shows schematically an embodiment of an apparatus for carryingout the method according to the invention, and

FIG. 2 illustrates the relative alignment of the pinhole diaphragms andthe zone plates used in this embodiment.

In the apparatus shown in FIG. 1 a beam of radiation emanating from asource of radiation, for example a laser, not shown, illuminates anarray of zone plates 3. This array produces a plurality of sub-beams,the individual zone plates focusing the sub-beams on the pinholediaphragms. To achieve satisfactory illumination of the pinholes thezone plates and the pinhole diaphragms are to be relatively aligned indifferent manners. This is shown by the arrows a to h in FIG. 2.

The spherical waves 6 which emerge from the matrix of pinhole diaphragms5 interfere with a reference wave 7 which has passed through an opticalsystem 8 and a pinhole diaphragm 9. The resulting interference patternis recorded in known manner as an amplitude hologram or a phasehologram.

According to the invention there is inserted in the path of theradiation beam 1, in front of the matrix of zone plates, an opticalimaging system 2. This imaging system focuses incident radiation on thephotographic plate 10. This ensures that that spherical waves whichemerge from the array of pinhole diaphragms cooperate with one anotherin an optimum manner, so that the information is uniformly distributedover the photographic plate 10.

Periodic arrays have the disadvantage that at predetermined distancesrepeated self-imaging planes occur which cause a highly irregularintensity distribution in the hologram plane. As a result, in thephotographic recording the linear range of the H and D curve may beexceeded. These inconvenient self-imaging planes may be suppressed inknown manner by statistically modulating the phase of the wave frontilluminating the periodic structure by means of a phase plate.

ln the arrangement of zone plates and pinhole diaphragms used in theembodiment described, according to the invention the front surface ofthe array of pinhole diaphragms 5 may be designed as a phase plate 4having a statistical distribution of the abrupt phase variations by thedeposition of dielectric materials from vapour. The magnitude of thesudden phase variations is determined by the thickness of the layer andby the index of refraction of the material.

The phase plate 4 introduces statistical phase variations in the variousradiation paths which prevent selfimaging as produced in periodicarrangements.

What is claimed is:

1. Apparatus for making multiple point holograms comprising a source ofcoherent radiation, a plate of photosensitive material, optical meansinterposed in the path of the coherent radiation for focussing saidradiation on the plate of photosensitive material, an array of pinholediaphragms located between the optical means and the photosensitivematerial in the path of the radiation passing through the optical means,an array of zone plates having a binary phase structure located betweenthe optical means and the array of pinhole diaphragms and arranged tofocus the radiation eminating from the optical means on the array ofpinhole diaphragms, the spacing of the array of zone plates being equalto that of the array of pinhole diaphragms,

and phase plate means located on said array of pinhole I diaphragms forrandomly varying the phase of the radiation eminating from the zoneplates and passing through the pinholes.

2. Apparatus as claimed in claim 1, wherein the phase plate meanscomprises a dielectric material vapour deposited on the array of pinholediaphragms.

1. Apparatus for making multiple point holograms, comprising a source ofcoherent radiation, a plate of photosensitive material, optical meansinterposed in the path of the coherent radiation for focussing saidradiation on the plate of photosensitive material, an array of pinholediaphragms located between the optical means and the photosensitivematerial in the path of the radiation passing through the optical means,an array of zone plates having a binary phase structure located betweenthe optical means and the array of pinhole diaphragms and arranged tofocus the radiation eminating from the optical means on the array ofpinhole diaphragms, the spacing of the array of zone plates being equalto that of the array of pinhole diaphragms, and phase plate meanslocated on said array of pinhole diaphragms for randomly varying thephase of the radiation eminating from the zone plates and passingthrough the pinholes.
 2. Apparatus as claimed in claim 1, wherein thephase plate means comprises a dielectric material vapour deposited onthe array of pinhole diaphragms.